1. Field of the Invention
The present invention relates to a liquid crystal display device comprising a liquid crystal display panel and a backlight, and in particular, to a liquid crystal display device which utilizes a light emitting diode (hereinafter simply referred to as “LED”) as a light source.
2. Description of Related Art
Among conventional liquid crystal display devices, transmissive-type and transflective-type liquid crystal display devices are each provided with a liquid crystal display panel and a backlight for supplying light transmitting through the liquid crystal panel.
Generally, a backlight includes a light source and a light-guiding plate, and a small fluorescent tube called CCFL (cold cathode fluorescent tube) is used as the light source. One principal surface of the light-guiding plate is disposed so as to correspond to a display area of the liquid crystal display panel, and a diffusion area for diffusing and reflecting light toward the surface side is provided on the other principal surface thereof (referred to as “outer surface”) opposite to the aforementioned principal surface.
The CCFL light source is disposed at an end surface of the light-guiding plate so that light of CCFL incident on the end surface of the light-guiding plate is transmitted inside the light-guiding plate, and is diffused/reflected on the outer surface side of the light-guiding plate to be directed from the surface of the light-guiding plate toward the liquid crystal panel. Thus, the light source is converted from a linear light source into a homogeneous planar light source to be utilized as the light source for the liquid crystal device.
However, this CCFL light source uses Hg (mercury) encapsulated in a discharge tube that emits ultraviolet rays when excited by electrical discharge, which strike the fluorescent substance on the CCFL tube wall to be converted into visible light rays.
For this reason, when considering the environmental aspect, using an alternative light source is required for restricting the use of hazardous mercury.
In addition, in order to illuminate the CCFL, a high-voltage and high-frequency switching circuit is necessary. However, since this causes high frequency noise, not only noise prevention is additionally required, but also problems such as slow light-up under low temperature, low luminous efficiency and the like are prone to arise.
In the meantime, as a new light source, an LED backlight utilizing a light emitting diode module (LED light source) accommodating LED chips characterized by a point light source has been developed.
With demands for lower price, higher luminous efficiency and environmental regulations, this backlight utilizing the LED light source is becoming to be widely used as the backlight for liquid crystal display panels.
At the same time, with increased brightness and display area of liquid crystal display devices, the demand for providing a plurality of LED light sources is increasingly high.
Accordingly, when an LED backlight is used for a high brightness, large size liquid crystal display panel, the LED light source, which is a point light source, needs to be converted into a planar light source that emits light rays evenly (light source that has been converted into even light rays at the light-emitting surface of the light-guiding plate). For this reason, it is necessary to adjust the material and structure of the diffusion area of the outer surface of the light-guiding plate, and to dispose the LED light source at an optimum position in accordance with the orientation of the LED light source.
However, one problem here is that the temperature of the LED and its peripheral regions rises due to heat generated from the LED chip, leading to decrease in luminous efficiency and life of the LED light source.
Therefore, in a liquid crystal display device provided with a LED backlight, reduction of heat storage in the mounting board on which the LED light source is mounted and suppression of temperature rise in the LED light source are required.